CHEM 1811 Lecture # 13
1
PRECIPITATION REACTIONS
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some ionic compounds dissolve and ionize in water
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cations and anions separate and become mobile
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these solutions conduct electricity
when different solutions are mixed, insoluble compounds may form
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solid formed this way is called a precipitate
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ppt may be separated by decanting or filtration
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precipitate-forming reactions are also called ionic displacement
phase notations (s) vs. (aq) are essential descriptors
e.g.
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(i.e. electrolytes)
K2CO3 (aq) + CaBr2 (aq) → CaCO3 (s) + 2 KBr (aq)
knowledge of solubility is needed to predict precipitation products
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now we treat substances as simply soluble or insoluble
(later we will see that a range of solubility exists)
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when two solutions are mixed, check both new ion pairs
CHEM 1811 Lecture # 13
2
AQUEOUS SOLUBILITY RULES (in order of precedence)
1)
alkali, ammonium, nitrate, acetate, hydrogen carbonate, chlorate,
and perchlorate salts
ARE SOLUBLE in water
2a)
most chloride, bromide, iodide and cyanide salts ARE SOLUBLE
EXCEPT with
Ag+ , Pb2+ , and Hg22+
2b)
most sulfate salts ARE SOLUBLE
EXCEPT with Ca2+ , Sr2+ , Ba2+ , Hg22+ , Ag+ , and Pb2+
3)
most hydroxide salts are INSOLUBLE
BUT
Ca(OH)2 Sr(OH)2 Ba(OH)2 are slightly soluble
4)
most oxide, sulfide , carbonate , chromate , phosphate , and
oxalate salts are INSOLUBLE
EXAMPLE
Circle the soluble ionic compounds in the following list.
AgNO3
SnCl2
Pb(CH3CO2)2
BaCrO4
SrSO4
Na3PO4
Fe(OH)3
CuI2
ZnS
Rb2CO3
(NH4)2C2O4
TiO2
AlBr3
Hg(ClO4)2
AgCl
Ca(NO2)2
WRITING IONIC EQUTIONS
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ppt rxns generally have (aq) reactants and one product (s)
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need to show phases
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mass and charge must balance
CHEM 1811 Lecture # 13
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3
3 WAYS to write equations for reactions between ions in water
1)
2)
3)
chemical (a.k.a. molecular) equation
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show molecular formulas or formula units
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good for overall balancing, stoichiometry
total (a.k.a. complete) ionic equation
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strong electrolytes are shown as separate ions
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compound stoich. may become rxn stoich.
net ionic equation
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eliminate redundant spectator ions
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best shows actual change
EXAMPLE
Writing ionic equations
Aqueous solutions of silver nitrate and potassium chloride are mixed
and a white ppt. forms; write molecular, total ionic, and net ionic equations.
molecular:
AgNO3 (aq) + KCl (aq) → AgCl (s) + KNO3 (aq)
total: Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq) → AgCl (s) + K+ (aq) + NO3- (aq)
Ag+ (aq) + Cl- (aq) → AgCl (s)
net:
NOTES:
always show charges in total and net ionic equ.
all equations must balance
CHEM 1811 Lecture # 13
EXAMPLE
4
Writing ionic equations
Write chemical, total and net ionic equations showing the reaction
between aqueous solutions of ammonium sulfate and barium bromide. Circle
the spectator ions in the total ionic equation.
(complete in class)
chem:
(NH4)2SO4 (aq) + BaBr2 (aq) →
total:
2 NH4+ (aq) + SO42- (aq) + Ba2+ (aq) + 2 Br- (aq)
net:
Ba2+ (aq) + SO42- (aq) → BaSO4 (s)
→